This invention relates to vehicle antilock brake control systems, and more specifically to a system and method for determining a vehicle reference speed in an antilock control system.
Wheel slip is a measure of the relative difference between vehicle speed and the speed of the wheel, and is represented by the equation: .lambda.=.sup.(V-.omega.) /.sub.v where .lambda. is wheel slip, v is vehicle speed and .omega. is wheel speed. Antilock brake control systems typically use the parameter of wheel slip in determining when and whether to take corrective action. Since wheel slip is a measure of the relative difference between vehicle speed and wheel speed, any inaccuracies in the determination of vehicle reference speed will result in inaccuracies in the calculated wheel slip. These inaccuracies degrade overall system performance. Therefore, it is important that the ABS controller have an accurate measurement of vehicle reference speed.
Most vehicle antilock control systems, because the vehicle is equipped with a wheel speed sensor at each wheel, use all or some combination of the driven wheels and undriven wheels to arrive at the reference speed. When there is little or no wheel slip present at the wheels, most known vehicle reference calculations encounter little difficulty with accurately estimating the true vehicle body speed from the wheel speeds. For example, a simple average of the four wheel speeds will produce an accurate estimate of the vehicle speed under steady state driving conditions. Relatedly, using the highest of the wheel speed values as the reference speed value is also accurate under steady-state conditions. However, as wheel slip increases on one or more of the wheels, the accuracy of a vehicle reference speed created from either the simple average or based upon the highest wheel speed begins to degrade. This is because, as each wheel develops slip and begins to slow more rapidly than the true vehicle body speed, an average of these wheel speeds produces an estimated vehicle speed which is lower than the actual vehicle body speed. Relatedly, the wheel with the highest speed may itself be operating in the unstable braking region which also produces an artificially low estimate of vehicle reference speed. Similarly, if the wheels are traveling over a rough road surface, such as gravel, pot holes, or chatter bumps, or when there is an intermittent connector short in the wheel speed sensor causing an erratic signal, the variations experienced in the wheel speed signals corrupt the vehicle reference speed value. Thus, a system which produces a vehicle reference speed by taking a simple average of the four wheel speeds or by using the highest of the four wheel speeds does not have a reference velocity estimation method which is robust.
Various methods have been proposed to overcome the difficulties associated with producing a robust estimate of vehicle reference speed. Some of these proposed solutions involve the use of auxiliary devices, such as vehicle body accelerometers, wind speed detectors, fifth wheels, and the like. However, the use of such auxiliary devices proves impractical in mass vehicle production situations, due not only to the cost factors but to the difficulty in installing and maintaining these auxiliary devices.
Other proposed methods which rely solely on the wheel speed information involve imposing predetermined boundaries beyond which vehicle reference speed is calculated using means other than the individual wheel speeds. For example, many systems have been known to set a vehicle deceleration limit at 1.0 g. If, after calculating vehicle speed using the individual wheel speed data, the vehicle reference speed is more than 1.0 g less than the reference speed calculated during the previous control cycle, these systems discard the calculated reference speed value and instead impose a reference speed value which is 1.0 g less than the previous calculated reference speed value. This predetermined bounding process works well if the vehicle is operating on a road surface which physically allows a vehicle to decelerate at 1.0 g. However, many surfaces, such as gravel, snow, wet asphalt and ice, do not generate sufficient wheel/road tractive forces to decelerate the vehicle at such a high rate. In these instances, the maximum physically possible vehicle deceleration may be as low as 0.1 g. Given that, a large reference speed deceleration threshold causes the reference speed value to decelerate at a rate much higher than is physically possible for the vehicle under those road conditions, which results in an unrealistically low reference speed value which in turns causes unrealistically low wheel slip values. This can result in overbraking of the wheels, which manifests itself as longer periods of operation in the unstable braking region which reduces braking effectiveness and reduces vehicle stability.
Also, the known bounding methods are rather noise sensitive. Since bounding is performed only to prevent gross unnecessary deceleration of the reference speed, a wheel speed value which is unrealistically high due to rough road or an intermittent sensor connector causes reference speed to also be unrealistically high. This causes wheel slips to be unrealistically high, which results in under-braking. Under-braking manifests itself as longer periods of below-optimal braking, reducing braking effectiveness and increasing stopping distances.
It is plain to see that, in order to produce a robust vehicle reference speed value using only the wheel speed information, simply bounding the deceleration rate will not produce the desired accuracy when the full range of road conditions and hardware failures are considered.
Ideally, an antilock system's reference speed calculation should be rather easy to implement, embodying the convenience of taking a simple average of the four wheel speeds, while possessing the robustness of more complex configurations. It should resist unnecessary deceleration and noise corruption, and it should do so without having to rely upon external devices. Such a system would then be both inexpensive to deploy and versatile and accurate enough to satisfy the performance needs of a variety of vehicle braking applications.